On the same hardware, 32 bit and 64 bit MS Windows should be very close in speed except under some circumstances having to do with making many many operating system calls.
64 bit versions of MATLAB can access much more memory, and that is becoming increasingly important.
However, R2015b is the last 32 bit version of MATLAB, so you should be switching over to 64 bit MATLAB unless you have code that will only work on 32 bit versions, such as if you need the Legacy interface of Data Acquisition Toolbox, or you need the Indeo5 compressor, or you need to access hardware for which only 32 bit interfaces are available.
At this time, Windows 10 is not advised if you need to use compilers, unless you are willing to go to R2015b and either a commercial compiler or the free MingW compiler. Installing versions of MATLAB before R2015b on Windows 10 can require some work-arounds.
